Generally, referring to FIGS. 5A and 5B, a rotation angle detection device is provided with a magnetism detection unit J1 (e.g., Hall element embedded in Hall IC) disposed at a rotation axis (named Z-axis), a magnetic flux supply magnet J2 which has a semi-cylinder shape and applies magnetic flux to the magnetism detection unit J1, and a magnetic flux attraction magnet J3 which has a semi-cylinder shape and attracts the magnetic flux applied to the magnetism detection unit J1 by the magnetic flux supply magnet J2.
The magnetic flux attraction magnet J3 and the magnetic flux supply magnet J2, which construct a magnetic flux generation unit, are fixed to the diametrically inner surface of a cylinder-shaped yoke J4. The magnetism detection unit J1 is surrounded by the magnetic flux attraction magnet J3 and the magnetic flux supply magnet J2.
When the relative rotation angle between the magnetic flux generation unit and the magnetism detection unit J1 varies, the density of the magnetic flux orthogonal to the magnetic detection surface of the magnetism detection unit J1 varies (referring to FIG. 2). The magnetism detection unit J1 outputs signals responding to the density of the magnetic flux orthogonal to the magnetic detection surface thereof.
According to the output signals from the magnetism detection unit J1, the rotation angle detection device detects the relative rotation angle between two members where the magnetic flux generation unit (including magnetic flux attraction magnet J3 and magnetic flux supply magnet J2) and the magnetism detection unit J1 are respectively fixed, for example, referring to U.S. Pat. No. 5,544,000, JP-2-122205A, U.S. Pat. No. 5,055,781, U.S. Pat. No. 4,893,502 and US-2004/0189288A1.
However, in this case, influences of a magnetic member (e.g., yoke J4) supporting the magnetic flux attraction magnet J3 and the magnetic flux supply magnet J2, a magnetic member at the vicinity of the rotation angle detection device, and the like may cause a bias (deviation) in the distribution of the magnetic flux, which is applied to the magnetism detection unit J1 by the magnetic-flux attraction magnet J3 and the magnetic flux supply magnet J2.
Specifically, referring to FIG. 6B, in the case where the Z-axis-direction left ends of the magnetic flux attraction magnet J3 and the magnetic flux supply magnet J2 are justified to that of the yoke 14, the magnetic flux generated by the magnetic flux generation unit leaks to the Z-axis-direction right side (of magnetic flux attraction magnet J3 and magnetic flux supply magnet J2) where no magnet is arranged. That is, there exists a magnetic flux leaking portion α at the Z-axis-direction right portion of the yoke J4.
Therefore, the magnetic flux applied to the magnetism detection unit J1 by the Z-axis-direction right ends of the magnetic flux attraction magnet J3 and the magnetic flux supply magnet J2 is abated. As a result, referring to FIG. 6C, the top of the distribution of the magnetic flux applied to the magnetism detection unit J1 deviates (in Z-axis direction) to the opposite side to the magnetic flux leaking portion α.
Therefore, in the case where the arrangement position of the magnetism detection unit J1 deviates in the Z-axis direction, the density of the magnetic flux applied to the magnetism detection unit J1 will significantly vary. Specifically, as shown in FIG. 6C, when the arrangement position of the magnetism detection unit J1 deviates in the range “A” in the Z-axis direction, the density of the magnetic flux applied to the magnetism detection unit J1 will vary in the large range “B”.
Because the density of the magnetic flux applied to the magnetism detection unit J1 will significantly vary responding to the Z-axis-direction arrangement error, the absolute accuracy of the rotation angle detection device is deteriorated.
When the design alteration of a part of the rotation angle detection device is required because of a mounting restriction (in vehicle, for example) or an purpose for cost reduction, the Z-axis-direction centers of the magnetic-flux attraction magnet J3 and the magnetic flux supply magnet J2 are to deviate from the magnetism detection unit J1 when being viewed in the Y-axis direction. In this case, the top of the distribution of the magnetic flux applied to the magnetism detection unit J1 deviates in the Z-axis direction.
For the sake of the malfunction restriction, it is necessary for the design of the rotation angle detection device to be significantly altered. Therefore, the cost is greatly increased due to the specification variation.